Bushing construction for encapsulated transformers



March 22, 1966 BUSHING CONSTRUCTION FOR ENCAPSULATED TRANSFORMERS Filed July 5I 1965 United States Patent Oflice 3,242,446 Patented Mar. 22, 1966 3,242,446 BUSHING CONSTRUCTION FOR ENCAPSULATED TRANSFORMERS Frank A. Leute, Zanesville, Ohio, assignor to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware Filed `uly 3, 1963, Ser. No. 292,567 1 Claim. (Cl. 336-96) This invention relates to encapsulated electrical transformers embedded in molded insulation.

Encapsulated electrical transformers are known wherein the transformer core and coil assembly is embedded in a resin jacket. Such encapsulated transformers may be constructed with integral projections which surround the conductors to the primary and secondary windings and accommodate terminal connections in a manner similar to insulating bushings on conventional transformers having metallic casings. Encapsulated transformers are often used outdoors, and such integral projections on the resin jacket do not provide maximum resistance to Weather. Further, the surface resistivity and other electrical characteristics of such integral projections are not satisfactory, for very high voltage transformers, and such electrical characteristics deteriorate when the encapsulated transformer having `said integral projections is continuously exposed to the weather. Encapsulated electrical transformers are also known having porcelain insulating bushings which are highly resistant to the weather and have anti-tracking, surface resistivity, and other electrical characteristics superior to those of the integral projections on the resin jacket. However, when a porcelain bushing on such an encapsulated transformer became cracked or broken during handling or use, it was heretofore sometimes necessary to scrap the entire transformer.

It is an object of the invention to provide an improved encapsulated transformer and a method of construction therefor which has porcelain insulating bushings that may be easily replaced if they are damaged.

This and other objects and advantages of the invention will be more readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein: v

FIG. l is a view in perspective of an encapsulated electrical transformer embodying the invention with a portion broken away to illustrate the construction of the coils; and

FIG. 2 is a section view taken longitudinally through one of the primary insulating bushings of the transformer of FIG. 1.

Referring to the drawing, an encapsulated electrical distribution transformer has a resin jacket 11 enclosing a transformer core and coil assembly which may include a closed magnetic core 12 having a pair of Winding legs surrounded by cylindrical coils 15 and connected by `upper and lower yokes 16. A mounting bracket 17 for transformer 10 may engage the portions of resin jacket 11 surrounding the yokes 16. Magnetic core 12 may be constructed of a plurality of magnetic ribbons of different Widths to provide a cruciform cross section, and the coils 15 may be wound on the straight Winding legs of the core 12 in a well-known manner. Each coil 15 may include a primary winding 18 separated by an insulating barrier 19 from a secondary winding 20 radially inward therefrom. Jacket 11 may Ibe of a suitable thermosetting encapsulating epoxy resin material to which a suitable hardener is added, and the jacket 11 may be formed by placing the core and coil assembly in a mold and then introducing the epoxy resin in liquid state into the mold. A plurality of secondary insulating bushings 21 extend outward from the portion of jacket 11 surrounding lower yoke 16 and are provided with metallic secondary terminal means 22 extending through jacket 11 and electrically connected to the low voltage windings 20. A pair of primary insulating bushings 23 constructed in accordance with the invention extend radially outward Ifrom portions of resin jacket 11 surrounding the coils 15.

Each primary insulating bushing 23 includes a conductor rod 24 extending through a boss 25 on the exterior surface or resin jacket 11. Conductor rod 24 is molded within jacket 11 and extends outwardly through boss 25. Conductor rod 24 has external threads 27 on its outer end and an axial opening 28 in its inner end in which the conductor of a cable 29 electrically connected to the primary winding 18 may be swaged. A porcelain shroud 30 having a plurality of petticoats 31 thereon and a conical axial bore 32 surrounds conductor rod 24 and rests upon the radially outer generally at surface of boss 25. During construction, .a coating 34 of a suitable release agent such as silicone grease which will not adhere to the epoxy resin material is coated on the internal surface of porcelain shroud 30 which delines the conical bore 32 and on the bottom surface 35 thereof which rests on boss 25. After porcelain shroud 30 is placed over conductor rod 24, `a suitable epoxy resin material in liquid state is introduced into the conical bore 32 in porcelain shroud 30 and cured. During curing the resin within porcelain shroud 30 solidies into a frustoconical mass, or projection 36 which adheres tenaciously to the surface of boss 25 but does not adhere to the coating of silicone grease 34. A metallic terminal cap 38 having a tapped hole 39 therein is threaded on the end of conductor rod 24 and rigidly clamps porcelain shroud 30 against boss 25 on resin jacket 11. Terminal cap 38 carries eyebolt connector means 41 for clamping a power line conductor (not shown).

If porcelain shroud 30 is cracked or otherwise damaged during handling, shipping, or use, the primary insulating bushing can be replaced without scrapping the entire encapsulated transformer as was sometimes required with prior art devices. Inasmuch as silicone grease coating 34 on the surfaces of porcelain shroud 30 defining the conical bore 32 and the bottom wall 35 does not adhere to the epoxy resin mass 36, it is only necessary to unscrew terminal cap 38 from conductor rod 24 and lift oif porcelain shroud 30 from the epoxy resin mass 36. In order to install a new bushing, a porcelain shroud 30 (partially shown in dotted lines) is provided having a conical bore 32' (shown in dotted lines) slightly larger in diameter Vthan bore 32; the bore 32 and the bottom wall of new porcelain shroud 30 are coated with silicone grease 34, the porcelain shroud 30 is placed over conductor rod 24 and conical epoxy resin projection 36; epoxy resin material in liquid state is introduced into the annular space between epoxy resin projection 36 and the conical bore 32 in new porcelain shroud 30; and the epoxy resin is cured. Terminal cap 38 is threaded on the end of conductor rod 24 to clamp porcelain shroud 30 against boss 25.

It will be appreciated that an encapsulated transformer embodying the invention has insulating bushings possess ing the highly desirable weather resistant, anti-tracking, and high surface resistivity characteristics of porcelain and which may easily be replaced in the event the porcelain becomes cracked or damaged, thus obviating the necessity of scrapping the entirev transformer as was required when a porcelain bushing of a prior art encapsulated transformer was damaged.

While only a single embodiment of the method and article of the invention has been illustrated and described, many modifications and variations thereof will be appar ent to those skilled in the art, and consequently it is intended in the appended claim 'to cover yall such modiications and variations thereof which come within the true spirit and scope of the invention.

I claim: l

In combination, a transformer core and coil assembly including a magnetic core linked by a Winding, a resin jacket surrounding said core and coil assembly, a conductor rod embedded in and extending through said jacket and being electrically connected to said winding, a porcelain tbushing having a.conical bore therein disposed over said rod and against said jacket, a coating of a release'agent on the surface of said bushing dening said conical bore yand on the surface thereof disposed against said resin jacket, a cured resin mass lling `said lbore in said resin jacket, and a metallic terminal cap releasably engaging said rod and clamping said bushing against said` jacket, whereby said porcelain' bushing may easily be replaced if it is damaged.

References Cited bythe Examiner UNITED STATES PATENTS 1,244,642 10/1917 Pruessman 336--107 X 1,816,023 7/ 1931 Osborne 264-264 X 3,001,005 y 9/1961 Sonnenberg 174-142 ROBERT K. SCHAEFER, Primary Examiner.

JOHN F. BURNS, Examiner.

surrounding relation to said rod and being adhered to 15 T. I. KOZMA, Assistant Examiner. 

